1. Field of the Invention
The present invention relates to an optical filter for use with, for example, a missile tracking system, and more particularly to an active resonant absorption cell for protecting a radiation detecting system from incident laser radiation; the absorption cell filled with a gas selected in accordance with the wavelength of the laser radiation to be filtered, such as deuterium fluoride (DF); the cell utilizing external resistance heating or optical pumping to raise the energy state of the gas within the cell to increase its absorption level to enable the incident laser radiation to be absorbed.
2. Description of the Prior Art
Various optical filters are known for use with different wavelengths. For example, in the 400-700 millimeter range, atomic resonant filters are known to be used as optical band pass filters to filter various types of noise, such as sunlight. Both active and passive atomic resonant filters are known. Such atomic resonant filters normally include a pair of spaced apart optical filters, used to form a hermetically sealed cell filled with an atomic vapor and an inert buffer gas. Active atomic resonant filters, for example, as disclosed in U.S. Pat. No. 4,829,597, are known to utilize lasers for raising the energy state of the atomic gas. Passive atomic resonant filters, for example, as disclosed in U.S. Pat. Nos. 5,081,361 and 5,311,369, are known to employ rubidium and cesium vapors while active resonant filters are known to employ sodium, thallium, rubidium, magnesium and calcium. In such atomic filters, the atomic vapor absorbs the photons from the incident radiation thereby increasing the energy state of the vapor. Collisions between the vapor in the raised energy state and the buffer gas further increase the energy state of the vapor causing the atomic vapor to return to a lower metastable energy level and emit radiation at a different wavelength.
U.S. Pat. No. 5,111,047, assigned to the same assignee as the assignee of the present invention, discloses an ionization type atomic resonant filter. Such a filter includes a chamber with a selective molecular vapor and an inert buffer gas. Incoming radiation is filtered by an optical filter. Photons from radiation received in the chamber raise the energy level of the molecular vapor to a metastable state. Collisions between the molecular vapor and the inert buffer gas cause ionization of a significant portion of a molecular vapor. An electrical field is used to collect free ions to produce an electrical current that is proportional to the incident radiation of interest.
Resonant absorption cells are also known for filtering laser radiation, for example, from a chemical laser, such as hydrogen fluoride (HF) and DF lasers for example, as described in detail in "High Energy-High Average Power Pulsed HF/DF Chemical Laser", by H. Burnet, M. Mabru and F. Voignier SPIE Vol. 2502 pgs. 388-392, hereby incorporated by reference. An example of such a resonant absorption cell is described in U.S. Pat. No. 4,880,978. Such an absorption cell includes a hermetically sealed chamber, two walls of which are formed by optical filters. The absorption cell is filled with a gas under pressure. A pair of electrodes is disposed within the cell and connected to a high voltage power supply (i.e. 25 kV) for generating an arc within the cell. The arc causes the gas molecules in the cell to be heated by plasma discharge thereby exciting the gas molecules to a relatively high energy state. By raising the energy state of the gas vapor, the absorption properties of the gas are increased to enable unwanted laser radiation to be absorbed.
Such filters as described above are used in various laser communication, imaging and tracking systems for example, as disclosed in U.S. Pat. Nos. 4,829,597; 5,079,414 and 5,181,135, for filtering radiation of various unwanted wavelengths. For example, the atomic resonant filters described above, are normally used to filter radiation in the 400-700 milliammeter range. The resonant absorption cells are used for filtering laser radiation for example, from HF/DF chemical laser and the high frequency range.
There are various drawbacks with the various optical filters discussed above. For example, the resonant absorption cell described above requires a hermetically sealed cell filled with a gas under pressure. As mentioned above the energy state of the gas is raised by plasma discharge which requires electrodes within the hermetically sealed cell connected to a power supply external to the cell; thus requiring hermetic sealing of the electrical conductors which penetrate the walls of the cell which increases the complexity as well as the cost of the cell. Additionally, a relatively high voltage power supply, 25 kV is required which further increases the complexity and cost of the cell.